A flexible wearable sensor for knee flexion assessment during gait

Abstract

Background: Gait analysis plays an important role in the diagnosis and management of patients with movement disorders but it is usually performed within a laboratory. Recently interest has shifted towards the possibility of conducting gait assessments in everyday environments thus facilitating long-term monitoring. This is possible by using wearable technologies rather than laboratory based equipment.

Research question: This study aims to validate a novel wearable sensor system's ability to measure peak knee sagittal angles during gait.

Methods: The proposed system comprises a flexible conductive polymer unit interfaced with a wireless acquisition node attached over the knee on a pair of leggings. Sixteen healthy volunteers participated to two gait assessments on separate occasions. Data was simultaneously collected from the novel sensor and a gold standard 10 camera motion capture system. The relationship between sensor signal and reference knee flexion angles was defined for each subject to allow the transformation of sensor voltage outputs to angular measures (degrees). The knee peak flexion angle from the sensor and reference system were compared by means of root mean square error (RMSE), absolute error, Bland-Altman plots and intra-class correlation coefficients (ICCs) to assess test-retest reliability.

Results: Comparisons of knee peak flexion angles calculated from the sensor and gold standard yielded an absolute error of 0.35(±2.9°) and RMSE of 1.2(±0.4)°. Good agreement was found between the two systems with the majority of data lying within the limits of agreement. The sensor demonstrated high test-retest reliability (ICCs>0.8).

Significance: These results show the ability of the sensor to monitor knee peak sagittal angles with small margins of error and in agreement with the gold standard system. The sensor has potential to be used in clinical settings as a discreet, unobtrusive wearable device allowing for long-term gait analysis.

Keywords: Gait analysis; Knee kinematics; Knee monitoring; Walking; Wearable technology.

Fig. 1

Markers and clusters positioning over the leggings, detail of the flexible sensor positioned anterior to the right knee and sensing node placed in the back pocket.

Fig. 2

Correlation and Bland Altman plot of agreement between knee peak angles measured by the sensor and MCS. Horizontal lines represent the mean difference and limits of agreement (dotted lines).